A major challenge in the development of novel anti-cancer strategies is the elucidation of molecular targets which are preferentially regulated in human tumors. Greater than 90% of all human cancer cells display robust activation of telomerase (Shay and Bacchetti, 1997), a unique reverse transcriptase consisting of two major components, the RNA moiety (hTR or hTERC) and the catalytic subunit (hTERT) (Feng et al., 1995; Greider and Blackburn, 1985; Lingner et al., 1995). In contrast, normal human cells are devoid of telomerase activity, with the exception of male germline cells and the proliferative cells of renewal tissues (Aisner et al., 2002; Forsyth et al., 2002; Shay and Wright, 2004). Telomerase activation is necessary for most cancer cells to replicate indefinitely and thereby enable tumor growth and subsequent metastasis.
In the last decade, a variety of methods have been devised to inhibit telomerase for cancer therapy (Gellert et al., 2005b; Resler et al., 2003; White et al., 2001). Oligonucleotide-based telomerase inhibitors that target the hTR component of telomerase have been described (Gryaznov et al., 2001). Preferred oligonucleotide-based telomerase inhibitors are N3′→P5′ thiophosphoramidate-linked (NPS) oligonucleotides, which have been shown to have a high thermodynamic duplex stability, as well as intracellular hydrolytic stability (Herbert et al., 2002).
The prototype NPS oligonucleotide telomerase inhibitor, having SEQ ID NO: 2 (TAG GGT TAG ACA A) and designated GRN163, causes reversible telomerase inhibition and subsequent telomere shortening in human mammary epithelial cells (Herbert et al., 2002). GRN163-induced telomere erosion also correlates with the induction of senescence or apoptosis in prostate cancer, multiple myeloma and non-Hodgkins lymphoma cells and a reduction of tumor growth in myeloma and glioblastoma xenograft models (Asai et al., 2003; Ozawa et al., 2004; Wang et al., 2004).
A second generation oligonucleotide-based telomerase inhibitor, designated GRN163L, includes a palmitoyl (C16) moiety conjugated to the 5′-terminus of GRN163 (Herbert et al., 2005). This lipid modification increases intracellular uptake, inhibition of telomerase, rate of telomere shortening, and growth inhibition and apoptosis in human mammary epithelial cell and hepatoma cell lines, relative to the unconjugated oligonucleotide (Djojosubroto et al., 2005; Herbert et al., 2005). GRN163L exhibits a multitude of anti-tumorigenic end points in lung and breast cancer cells (Dikmen et al., 2005; Gellert et al., 2005a). In vivo, the compound diminishes the tumor burden of metastatic human lung cancer and orthotopic human breast cancer cells in xenograft mouse models (Dikmen et al., 2005; Gellert et al., 2005a). The activity of GRN163L as an anti-cancer agent has been correlated with its observed ability to inhibit telomerase activity, in a dose- and sequence-dependent manner, via direct interaction with hTR.